CW, the problem is if you do exercises incorrectly you'll have to re-train the horse anyway. I'd just keep riding the way you do, do lots of transitions (walk - trot, trot - canter, etc.), may be some uphill work (if you have hills) to build the top, etc.
Also "101 Dressage Exercises" seems to be a very nice book. I looked at it in Dover just other day, thinking about ordering it.
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